This invention relates to glare reducing rearview mirror assemblies for use in vehicles and, more particularly, to an electrochromic/electrochemichromic rearview mirror assembly having spaced substrate panels which provides reduced glare reflection and thus improved glare reduction as well as substantially less noticeable double imaging in both interior and exterior vehicle applications.
Recent advancements in rear vision systems for vehicles include electro-optic rearview mirrors having an electrically activated medium which darkens to reduce glare in response to bright lights sensed from the rear of the vehicle. Although single substrate forms of such rearview mirrors are known, a more typical, conventionally known version is shown at 10 in FIG. 1. Prior known rearview mirror 10 consists of two planar substrates 11, 12 which are usually formed from glass although other optically transparent panels such as polycarbonate, acrylic and the like can also be used. Panels 11, 12 are spaced apart by a sealing material 14 defining a cell cavity 15 between the rear surface of front panel 11 and the front surface of rear panel 12. Transparent, electrically conductive coatings 16, 17, usually indium-tin oxide or an equivalent, are typically coated on those facing surfaces prior to assembly. In addition, the rearmost surface 19 of substrate 12 typically includes a reflector coating 20 of silver, aluminum, chromium or like metal, followed by a scattering preventing layer 21 of tape, plastisol or the like.
In this conventionally known assembly, cavity 15 is typically filled with an electro-optic material 18 such as a liquid crystal, liquid or gel electrochromic or like medium. When no electrical voltage is applied across electrical leads 22, 24 which are connected to conductive layers 16, 17, the electro-optic material is optically clear and transparent. Hence, incident light (I.sub.o) enters through substrate panel 11, passes through cell cavity 15, electro-optic material 18, transparent conductive coatings 16, 17, rear panel 11 and reflects off layer 20 after which it is retransmitted through electro-optic medium 18, rear and front substrates 12, 11 and coatings 16, 17 to form reflected image R. Typically, reflected image R is 50 to 90% of the incident light intensity I.sub.o depending on the nature and quality of reflective layer 20. However, when suitable electric voltage is applied to leads 22, 24, electro-optic medium 18 in cavity 15 is darkened such that incident light I.sub.o is attenuated by that medium, is reflected off layer 20, and is further attenuated in intensity during its second pass through medium 18 and out through front panel 11 to form dimmed image R' . Thus, when voltage is applied to electro-optic medium 18, the attenuated reflected image R' is usually 1 to 5% of the intensity of incident light I.sub.o.
In addition to the principal full or dimmed reflected image R or R', an observer of rearview mirror assembly 10 will also see a second image FR due to reflection from the front surface 13 of front substrate 11. First surface reflection FR is a natural consequence of the difference between the refractive index of the incident medium, in this case air with an index of 1, and the refractive index of the material from which the surface 1 is composed, in this case soda lime glass having a refractive index of about 1.52 (measured at sodium D line). The reflectivity R for a bare dielectric surface such as soda lime glass at normal (as opposed to oblique) incidence is given by the Fresnel coefficient: ##EQU1## where n.sub.o is the refractive index of the incident medium, which is usually air, and n.sub.s is the refractive index of the substrate. If light is obliquely incident, then the above equation still applies if the appropriate effective index is used in place of n.sub.o and n.sub.s. Thus, first surface reflection FR from such a glass substrate is about 4% using the above equation. Total reflectivity TR as seen by an observer of assembly 10 will include the sum of the first surface reflection FR and either the full or dimmed mirror reflection R or R', depending on whether the electro-optic mirror is unactivated and dimmed.
Use of known electro-optic rearview mirrors such as assembly 10 creates three problems with respect to the viewed reflection. First, no matter how dark the electro-optic material becomes, total reflectivity TR cannot be less than 4%, i.e., the 4% from first surface reflection FR. Hence, very intense headlights or similar intense light sources around the vehicle in which the rearview mirror system is being used, have the potential to continue to at least partially produce vision impairing glare despite even total elimination of the reflectance from the mirror reflector 20 due to dimming of electro-optic material 18 in cavity 15.
Secondly, if front surface 13 and rear surface 19 of substrate 12 on which reflector layer 20 is placed are not perfectly parallel, then the first surface reflectance image FR will not superimpose with the reflected image from mirror surface 20. Thus, a double image is formed where an observer would see both R or R' and FR. This double image will be particularly noticeable when the two images are close in intensity, i.e., when FR is about 4% and when R' is between 1 and 5% as is common when electro-optic material 18 is activated to its dimmed state.
Thirdly, if glass substrates 11, 12 are relatively thick, i.e., 2 millimeters or so, the double image of FR and R or R' will be particularly discernible when the assembly is viewed from an oblique angle.
Of course, other surfaces such as the rear surface of front panel 11 and the front surface of rear panel 12 can potentially contribute reflections in addition to first surface reflection FR and mirror reflection R or R'. However, the refractive index of electro-optic material 18, along with the refractive index of any coatings 16, 17 used on those additional surfaces, can be specified to be sufficiently close to that of substrates 11, 12 so as to sufficiently weaken the intensity of such additional reflectance to avoid problems from additional double imaging or significant contribution to overall total reflectance TR.
In addition to the above, it has been discovered that the above problems are more pronounced when front substrate panel 11 is replaced with a laminate front assembly for safety and other reasons as described in co-pending, commonly assigned, U.S. patent application Ser. No. 07/464,888, filed by me on Jan. 16, 1990, entitled "ANTI-SCATTER, ULTRAVIOLET PROTECTED, ANTI-MISTING, ELECTRO-OPTICAL REARVIEW MIRROR," the disclosure of which is hereby incorporated by reference. In such an assembly, inclusion of two adhered or laminated substrate panels in front of the electro-optic medium creates an additional opportunity for slight misalignment or nonparallelism between the front surface of the assembly and the mirror reflectance surface somewhere to the rear in the assembly. Hence, the problems of less efficient glare reduction and double imaging can be even more pronounced.
In addition, inefficient glare reduction and double imaging can also be more pronounced in electrochromic or electrochemichromic mirror assemblies using a liquid or gel medium in the cavity between the spaced substrates as in FIG. 1 as contrasted with solid-state, single substrate electrochromic assemblies or light scattering liquid crystal mirror assemblies. In the solid-state, single substrate assemblies, the parallel relationship of the front surface and the rear reflector surface can be easily controlled when forming the single substrate. In the liquid crystal assemblies, light is scattered when the liquid crystal layer is activated, as opposed to being absorbed by a darkened layer, tending to reduce the glare and double imaging more naturally.
Prior efforts to solve these glare and double imaging problems have been made such as in Japanese Patent Publication 61-7803 of Jan. 14, 1986, entitled "NONGLARING TYPE REFLECTING MIRROR." Such assembly provides a prismatic, laminated front glass piece in a liquid crystal mirror assembly where the front surface is purposely set at an angle to the rear surface of the front substrate. In such case, front surface reflected image FR is diverted at a significant angle with respect to the principal reflected image R to avoid the double imaging described above. This concept of purposeful divergence of reflected images using a prismatic element is well-known in mechanical day/night type rearview mirrors.
However, the prismatic front piece in electro-optic mirror assemblies has several drawbacks. Such prismatic front piece assemblies are difficult to use on outside mirrors due to the viewing angles involved and increase the overall device weight and thickness due to the need for a wedge-shaped glass element. In addition, prismatic front pieces are impractical for use on mirrors of compound curvature such as convex outside mirrors now commonly installed on vehicles for increased viewing range, and are very expensive to manufacture, especially for large area mirrors such as outside truck mirrors, and for mirrors requiring a variety of mirror shapes. Varying shapes would require a wide range of individual prismatic elements and necessitate large fabrication costs
Accordingly, the need was apparent for an improved electro-optic rearview mirror assembly, especially of the electrochemichromic variety, wherein the problems of low end reflectance limitations and of double imaging found in prior known mirror assemblies are reduced or eliminated.